Various types of lighting equipment are used for providing light in the nighttime or in an indoor space and for spotlighting objects. Such lighting equipment provides light by receiving electrical energy from an electrical power source and converting it to light energy, where incandescent light bulbs or fluorescent tubes are generally used. However, incandescent light bulbs and fluorescent tubes entail high power consumption and heat emission, and due to their short durability of about six months, there is a need for frequent replacement.
In addition, fluorescent tubes use mercury, which is a carcinogenic substance, and may thus be subject to regulations, and when fluorescent tubes are used for direct lighting, the flickering of the light source may cause weakened eyesight. There are also several other drawbacks, including the requirement of large installation spaces, and difficulties in installation and color adjustment, etc.
As an alternative to such conventional lighting devices, much research has been focused on lighting devices using LED's (light emitting diodes). An LED has a semi-permanent durability of about one hundred thousand hours, and since it uses light having a longer wavelength than that of ultraviolet rays, it is harmless to the human body.
LED's can be grouped, according to the wavelength of the light emitted, into blue LED's, white LED's, and ultraviolet LED's, etc. However, ultraviolet LED's and white LED's not only require complicated fabricating processes, but also incur much higher fabricating costs, such that their use is substantially limited. Thus, there is being developed a photoluminescent sheet for implementing white colors with inexpensive blue LED's. Such lighting equipment may be used for a backlight unit in a cell phone, etc.
However, since the conventional photoluminescent sheet is formed by kneading an inorganic phosphor with a thermoplastic resin as the matrix resin using a two-axis extruder, it is difficult to obtain uniform mixing of the phosphor in the resin sheet, which causes severe non-uniformity in luminance and color coordinates.
Also, since the conventional color conversion sheet was fabricated by extrusion molding a colored pellet, in which a matrix resin and a phosphor were kneaded, identical extrusion-molding conditions produced only identical forms of color conversion sheet, to result in low applicability.
Furthermore, thermoplastic resins such as polycarbonate or polymethylmethacrylate resin, etc., used for the matrix resin were limited in application, because they are high in brittleness and may easily break, as in the photograph of FIG. 1, and because they are low in heat-resistance (120° C. or lower) and may easily undergo plastic deformation at high temperatures.